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Synthesis, crystal structures, andsuperoxide dismutase activity oftwo isostructural copper(II)–zinc(II)complexes derived from N,N′-bis(4-methoxysalicylidene)cyclohexane-1,2-diamineZhong-Lu You a , Li-Li Ni a , Peng Hou a , Ji-Cai Zhang a & CheWang aa Department of Chemistry and Chemical Engineering , LiaoningNormal University , Huanghe Road 850#, Dalian 116029, P.R. ChinaPublished online: 18 Jan 2010.

To cite this article: Zhong-Lu You , Li-Li Ni , Peng Hou , Ji-Cai Zhang & Che Wang (2010) Synthesis,crystal structures, and superoxide dismutase activity of two isostructural copper(II)–zinc(II)complexes derived from N,N′-bis(4-methoxysalicylidene)cyclohexane-1,2-diamine, Journal ofCoordination Chemistry, 63:3, 515-523, DOI: 10.1080/00958970903515134

To link to this article: http://dx.doi.org/10.1080/00958970903515134

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Journal of Coordination ChemistryVol. 63, No. 3, 10 February 2010, 515–523

Synthesis, crystal structures, and superoxide dismutase

activity of two isostructural copper(II)–zinc(II)

complexes derived from N,N0-bis(4-methoxysalicylidene)

cyclohexane-1,2-diamine

ZHONG-LU YOU*, LI-LI NI, PENG HOU,JI-CAI ZHANG and CHE WANG

Department of Chemistry and Chemical Engineering, Liaoning Normal University,Huanghe Road 850#, Dalian 116029, P.R. China

(Received 14 February 2009; in final form 21 September 2009)

Two new isostructural copper(II)–zinc(II) complexes, [CuZnLBr2] (1) and [CuZnLCl2] (2)(H2L¼N,N0-bis(4-methoxysalicylidene)cyclohexane-1,2-diamine), have been synthesized andcharacterized by elemental analyses, infrared spectroscopy, and single-crystal X-ray diffraction.Both complexes crystallize in the P-1 space group. The Cu in each complex is four-coordinatesquare planar with two imines and two phenolates of L. The Zn in each complex isfour-coordinate tetrahedral with two phenolates of L and two halides (Br for 1 and Cl for 2).The superoxide dismutase (SOD) activity of the complexes indicates that both complexes arerudimentary models for SOD.

Keywords: Schiff base; Copper; Zinc; Crystal structure; Superoxide dismutase

1. Introduction

Dinuclear complexes particularly heterobimetallic systems are of special interestbecause of their relevance to physics, chemistry, and biology [1]. Superoxide dismutase(SOD) is an important enzyme in biological systems, which catalyzes the dismutationof the superoxide radical to hydrogen peroxide and molecular oxygen [2, 3].Crystallographic studies show that the Cu–Zn SOD contains an imidazolato-bridgedcopper(II) and zinc(II) in the active site [4]. Coordination is distorted square pyramidalfor copper(II) and tetrahedral for zinc(II). Considering that heterodinuclear Cu–Zncomplexes are very rare to date [5–7], in this article, two isostructural dinuclear Cu–Zncomplexes, [CuZnLBr2] (1) and [CuZnLCl2] (2) (H2L¼N,N0-bis(4-methoxysalicylidene)cyclohexane-1,2-diamine), have been synthesized and structurally characterized.

*Corresponding author. Email: youzhonglu@yahoo.com.cn

Journal of Coordination Chemistry

ISSN 0095-8972 print/ISSN 1029-0389 online � 2010 Taylor & Francis

DOI: 10.1080/00958970903515134

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The SOD activity of the complexes was tested. To our knowledge, the complexesderived from the Schiff-base H2L have not been reported.

N N

OH OH

O O

1 2

43

875 6

9

10

1112

13

14

15

1617

18

1920

21

22H2L

2. Experimental

2.1. Materials and methods

All chemicals (reagent grade) were commercially available and used without purifica-

tion. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240Celemental analyzer. Infrared (IR) spectra were recorded on a Nicolet AVATAR 360

spectrometer as KBr pellets from 4000 to 400 cm�1. 1H NMR spectra were recorded ona Mercury Vx300 spectrometer (300MHz) at room temperature using CDCl3 assolvent and tetramethylsilane as internal standard. Molar conductance was measured

with a Shanghai DDS-11A conductometer. Electronic absorption spectra wererecorded on acetonitrile solutions of the complexes on a Shimadzu UV-Vis 160spectrophotometer.

2.2. Synthesis of H2L

To methanol solution (50 cm3) of 4-methoxysalicylaldehyde (152.1mg, 1.0mmol) was

added a methanol solution (30 cm3) of cyclohexane-1,2-diamine (57.1mg, 0.5mmol)with stirring. The mixture was stirred for 10min at room temperature to give a yellowprecipitate. The product was filtered, washed three times with cold methanol, and dried

in air. Yield: 96%. Anal. Calcd for C22H26N2O4 (%): C, 69.1; H, 6.8; N, 7.3. Found:C, 68.7; H, 6.9; N, 7.2. 1H NMR (CDCl3, 300MHz) (ppm): �¼ 12.03 (b, OH); 8.23

(s, 2H, CH¼N); 7.23 (d, 2H, Ar–H); 6.45 (d, 2H, Ar–H); 6.33 (s, 2H, Ar–H); 3.72(s, 6H, OCH3); 3.37 (m, 2H, chiral H); 1.83–1.40 (m, 8H, (CH2)4).

13C NMR (CDCl3,300MHz) (ppm): �¼ 22.3 (C1, C2), 29.7 (C3, C4), 55.3 (C15, C22), 62.3 (C5, C6), 98.7

(C11, C20), 104.5 (C13, C18), 118.2 (C9, C16), 127.7 (C14, C17), 160.3 (C10, C21),164.1 (C7, C8), 167.2 (C12, C19).

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2.3. Synthesis of 1

To methanol solution (10 cm3) of H2L (38.2mg, 0.1mmol) was added a methanolsolution (10 cm3) of Cu(ClO4)2 � 6H2O (37.0mg, 0.1mmol) and ZnBr2 (22.5mg,0.1mmol) with stirring. The mixture was stirred for 30min at room temperatureto give a clear blue solution. After keeping the solution in air for a week, blueblock-shaped crystals of 1, suitable for X-ray crystal structural determination, formedat the bottom of the vessel on slow evaporation of the solvent. The crystals wereisolated, washed three times with methanol, and dried in air. Yield: 72%. Anal. Calcdfor C22H24Br2CuN2O4Zn (%): C, 39.5; H, 3.6; N, 4.2. Found: C, 40.1; H, 3.8; N, 4.0.

2.4. Synthesis of 2

Complex 2 was synthesized by similar method as described for 1, with ZnBr2 replacedby ZnCl2 (13.7mg, 0.1mmol). Blue block-shaped crystals of 2, suitable for X-ray crystalstructural determination, were formed at the bottom of the vessel on slow evaporationof the solvent. The crystals were isolated, washed three times with methanol, and driedin air. Yield: 63%. Anal. Calcd for C22H24Cl2CuN2O4Zn (%): C, 45.5; H, 4.2; N, 4.8.Found: C, 45.0; H, 4.2; N, 4.7.

2.5. X-ray structural determination

Diffraction intensities for 1 and 2 were collected at 298(2)K using a Bruker SMARTCCD area detector with Mo-Ka radiation (�¼ 0.71073 A). The collected data werereduced using SAINT [8] and empirical absorption corrections were performed usingSADABS [9]. The structures were solved by direct methods and refined against F2 byfull-matrix least-squares using the SHELXTL package [10]. All non-hydrogen atomswere refined anisotropically. H atoms were placed in calculated positions andconstrained to ride on their parent atoms. The crystallographic data for the twocomplexes are summarized in table 1. Selected bond lengths and angles are given intable 2.

2.6. SOD measurement

The SOD activity (the concentration of the complexes corresponding to 50% inhibition)of the complexes was measured by the Beauchamp–Fridovich assay [11]. Superoxideanions were produced from the riboflavin/methionine system. The indicator utilized inthis case is nitroblue tetrazolium (NBT), which reacts with O�2 to form blue formazane.Cells containing 3.3� 10�6M riboflavin, 0.01M methionine, 4.6� 10�5M NBT, and0.05M phosphate buffer pH 7.4, and 10�7–10�6M complex or 2� 10�9–2� 10�8Mnative Cu–Zn SOD, were illuminated under fluorescent lamps. The absorbance at560 nm increased linearly with time of illumination. The reduction of NBT wasmeasured in terms of increased absorbance at 560 nm on a Shimadzu UV-240spectrophotometer. All photo-induced reactions were performed at 25�C. Each test wasperformed in triplicate.

Copper(II)–zinc(II) complexes 517

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Table 1. Crystallographic and experimental data for 1 and 2.

Complex 1 2

Emperical formula C22H24Br2CuN2O4Zn C22H24Cl2CuN2O4ZnFormula weight 669.16 580.24Temperature (K) 298(2) 298(2)Crystal shape/color Block/Blue Block/BlueCrystal system Triclinic TriclinicSpace group P-1 P-1Unit cell dimensions (A, �)a 7.888(1) 7.881(1)b 11.980(2) 11.676(1)c 12.877(2) 12.794(1)� 81.656(2) 81.262(2)� 83.758(3) 84.263(2)� 78.728(3) 79.094(2)Volume (A3), Z 1176.7(3), 2 1139.5(2), 2Calculated density (g cm�3) 1.889 1.691Absorption coefficient m (Mo-Ka) (mm�1) 5.350 2.252F(000) 662 590Crystal size (mm3) 0.30� 0.30� 0.28 0.21� 0.20� 0.18Measured reflections 6713 6838Independent reflections 4893 [R(int)¼ 0.0243] 4775 [R(int)¼ 0.0110]Observed reflections [I� 2�(I )] 3313 4052Restraints 0 0Parameters 291 291Min. and max. transmission 0.297 and 0.316 0.649 and 0.687Goodness-of-fit on F2 1.037 1.053Final R indices, [I� 2�(I )]a R1¼ 0.0435, wR2¼ 0.0941 R1¼ 0.0287, wR2¼ 0.0716R indices (all data)a R1¼ 0.0760, wR2¼ 0.1059 R1¼ 0.0359, wR2¼ 0.0753Largest difference peak and hole (e A�3) 0.729 and �0.566 0.493 and �0.494

aR1¼Fo�Fc/Fo, wR2¼ [P

wðF 2o � F 2

c Þ/P

w(F 2o )

2]1/2, w1¼ [�2ðF 2o Þ þ (0.0452ðF 2

o þ 2F 2c Þ/3)

2]�1, w2¼ [�2ðF 2c Þ þ (0.0373ðF 2

oþ

2F 2c Þ/3)

2þ 0.4426ðF 2

o þ 2F 2c Þ/3]

�1.

Table 2. Selected bond lengths (A) and angles (�) for 1 and 2.

1

Cu1–O1 1.914(3) Cu1–O2 1.893(3)Cu1–N1 1.896(3) Cu1–N2 1.907(4)Zn1–O1 2.030(3) Zn1–O2 2.027(3)Zn1–Br1 2.322(1) Zn1–Br2 2.323(1)Cu1–Zn1 2.998(1)O2–Cu1–N1 171.7(2) O2–Cu1–N2 95.5(2)N1–Cu1–N2 86.3(2) O2–Cu1–O1 83.4(2)N1–Cu1–O1 95.9(2) N2–Cu1–O1 171.9(2)O2–Zn1–O1 77.3(2) O2–Zn1–Br1 112.2(1)O1–Zn1–Br1 112.2(1) O2–Zn1–Br2 112.7(1)O1–Zn1–Br2 117.7(1) Br1–Zn1–Br2 117.9(1)

2

Cu1–O1 1.898(2) Cu1–O2 1.912(2)Cu1–N1 1.913(2) Cu1–N2 1.901(2)Zn1–O1 2.035(2) Zn1–O2 2.040(2)Zn1–Cl1 2.193(1) Zn1–Cl2 2.193(1)Cu1–Zn1 3.003(1)O1–Cu1–N2 170.5(1) O1–Cu1–O2 83.7(1)N2–Cu1–O2 96.0(1) O1–Cu1–N1 95.5(1)N2–Cu1–N1 86.4(1) O2–Cu1–N1 170.4(1)O1–Zn1–O2 77.2(1) O1–Zn1–Cl2 113.4(1)O2–Zn1–Cl2 111.0(1) O1–Zn1–Cl1 111.9(1)O2–Zn1–Cl1 118.2(1) Cl2–Zn1–Cl1 118.3(1)

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3. Results and discussion

Both complexes were synthesized by reaction of H2L with copper(II) perchlorate andzinc(II) halide. The perchlorate anion does not coordinate, leading to formation ofsquare planar Cu. The phenolate O atoms are good bridging groups [12, 13] and furthercoordinate to Zn forming dinuclear complexes. The halides complete the tetrahedralcoordination of Zn. Both isostructural complexes are stable at room temperature,

soluble in methanol, ethanol, and acetonitrile.

3.1. Spectroscopic study

IR spectra of H2L and the two complexes provide information about metal–ligandbonding. Assignments are based on typical group frequencies. Weak and broadabsorptions at 3433 cm�1 substantiate the presence of phenol in H2L, which disappearin the complexes. The strong absorption at 1637 cm�1 in the spectrum of H2L assigned

to the azomethine �(C¼N) [14] shifts to lower wave number in the complexes,1617 cm�1 for 1, and 1616 cm�1 for 2, attributed to coordination of nitrogen to Cu. Thestrong absorption of Ar–O in H2L at 1232 cm�1 [15] is located at lower frequenciesfor the complexes, namely 1203 cm�1 for 1 and 1202 cm�1 for 2. Weak absorptionsindicative of Zn–Br in 1 and Zn–Cl in 2 are at 246 and 293 cm�1, respectively.

Both complexes show two typical absorption bands at about 640 and 490 nm in the

visible electronic spectra. The first corresponds to typical d–d band in a square planarcoordination around copper(II), which is in agreement with the crystal structures ofthe complexes. The second could be attributed to a ligand-to-metal charge transfertransition (LMCT).

3.2. Structure description of 1 and 2

The single-crystal X-ray diffraction shows that 1 and 2 are isostructural dinuclear

copper(II)–zinc(II) complexes (figures 1 and 2). The dihedral angles between thecorresponding benzene rings are 25.3(3)� in 1 and 27.0(3)� in 2. Both cyclohexyl rings inthe complexes adopt chair conformations. The Cu � � �Zn distances are 2.998(1) and3.003(1) A in 1 and 2, respectively.

The Cu in each complex is coordinated by two imine nitrogens and two phenolateoxygens of L, forming a slightly distorted square plane. The two trans angles are 171.7(2)�

and 171.9(2)� in 1, and 170.4(1)� and 170.5(1)� in 2. The Cu lies in the least-squaresplane. The coordinate bond lengths related to Cu are comparable to each other and alsocomparable to corresponding values observed in similar complexes [16–18].

The Zn in each complex is coordinated by two phenolate oxygens of L and twohalides (Br for 1 and Cl for 2), forming a slightly distorted tetrahedral coordination.The Br1–Zn1–Br2 plane in 1 and the Cl1–Zn1–Cl2 plane in 2 are nearly perpendicularto the CuL units, which can decrease the steric effects between them. The Zn–N bond

lengths in both complexes are comparable to each other, but the Zn–Br bonds in 1 aremuch longer than the Zn–Cl bonds in 2. All the coordinate bond lengths arecomparable to corresponding values observed in similar complexes [16–18].

Copper(II)–zinc(II) complexes 519

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3.3. SOD activity

Schiff-base metal complexes have been widely investigated [19, 20]. Most Schiff basesand their complexes were investigated for biological activities, such as antibacterial[21, 22], cytotoxic [22], antifungal [23], and urease inhibition [24]. Further, a number ofcopper(II) complexes with Schiff bases have been investigated for their SOD activities[23, 25, 26]. However, Cu–Zn complexes with Schiff bases have seldom been reportedfor SOD activities.

The SOD activities of the two complexes in the present work are similar. Thepercentage of inhibition against complex concentration was plotted in figure 3.

Figure 1. The structure of 1, showing the atom-numbering scheme. Displacement ellipsoids are drawn at30% probability and H atoms are shown as small spheres of arbitrary radii.

Figure 2. The structure of 2, showing the atom-numbering scheme. Displacement ellipsoids are drawn at30% probability and H atoms are shown as small spheres of arbitrary radii.

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The values corresponding to IC50 were 3.506 mM for 1 and 3.127mM for 2. NativeCu–Zn SOD was used as a reference, with IC50 of 0.005 mM. Although the twocomplexes showed SOD activity, they are much weaker than native Cu–Zn SOD. Thedifference of the SOD activities among the complexes and the native SOD results fromstructural features. It is well-known that the active center of the native Cu–Zn SODconsists of copper(II) and zinc(II) ions bridged by a histidyl imidazolate anion. Onewater and three L-histidine molecules further coordinate copper(II) ion. Beside theimidazolato bridge the zinc(II) ion is bonded to an L-aspartic acid and two L-histidinemolecules. The two complexes reported in this article are also Cu–Zn heterometalliccomplexes; however, the copper(II) and zinc(II) ions are bridged by two phenolate Oatoms, and there are many other differences when compared with the native Cu–ZnSOD. Even though the native Cu–Zn SOD shows excellent activities, it is quitesensitive. Changes in temperature, solvating properties, etc. may easily lead todenaturation (frequently irreversibly). The two complexes reported in this article arevery stable in solutions, the molar conductance values of the complexes measured indistilled water (containing small quantity of DMSO) at the concentration of1.0� 10�3mol L�1 are 15.7 and 17.2mS cm�1 for 1 and 2, respectively, indicating thenon-electrolytic nature of the complexes [27].

For comparison, the IC50 values of some copper(II) complexes and the native Cu–ZnSOD were also summarized in table 3. It can be seen that the IC50 values of the two

01 2 3 4 5 6

Concentration (µM)

10

20

30

40

50

60

70

80

Inhi

bitio

n (%

)

Figure 3. Percentage inhibition of NBT reduction plotted against the concentration of the complexes(g for 1, m for 2). Each point represents the mean� SD of triplicate determination.

Table 3. SOD activities as IC50 for test materials.

Complexa IC50 (mM) Reference

1 3.506 This work2 3.127 This work[Cu(C9H7NO3)(C12H8N2)] 6.15 [26][Cu(N-meim)6](sal)2 0.17 [28][Cu(en)2](sal)2 3.16 [29][Cu(Pu-6-MePy)(H2O)](ClO4)2 2.25 [30]SALCuCl2 3.9 [31]Cu–Zn SOD 0.005 This work

aC9H7NO3¼ salicylideneglycine; C12H8N2¼ 1,10-phenanthroline; N-meim¼N-methylimidazole;sal¼ salicylate; en¼ ethylenediamine; SALH2¼ 40,50-bis(salicylideneimino)benzo-15-crown-5.

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complexes in this article are comparable or superior to those reported in the literature;however, they are significantly less active than the native enzyme. Nevertheless, the twocomplexes are potent SOD mimics considering their very low molecular weightcompared with that of the native enzyme.

4. Conclusions

Two isostructural heterodinuclear copper(II)–zinc(II) complexes were synthesized andstructurally characterized. Both complexes showed weak SOD activities, but are muchmore stable in solutions than native Cu–Zn SOD. Further work needs to be done toimprove the mimic models of the native Cu–Zn SOD.

Supplementary material

Crystallographic data for the complexes have been deposited with the CambridgeCrystallographic Data Center (CCDC 720314 for 1 and 720315 for 2).

Acknowledgements

This work was financially supported by the Education Office of Liaoning Province(Project No. 2008T107).

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